Veins

A vein is a blood vessel that carries deoxygenated blood to the heart.

Compared to an artery, a vein has a wider lumen that can measure from 0.5 mm to 3 cm in diameter.

Additionally, its tunica interna is thinner, consisting of a thin endothelium. The tunica media is poorly developed, with fewer smooth muscle and elastic fibers. The tunica externa is well-developed and comprises longitudinally arranged collagen and elastic fibers.

Veins also lack internal and external elastic laminae.

Many veins, particularly those in the limbs, are equipped with valves — thin folds of tunica interna that form flaplike cusps. These valves project into the lumen and are oriented to prevent the backflow of blood.

A venous or vascular sinus is a specialized vein that appears flattened as it has only an extremely thin layer of endothelium, lacking all other tunics.

Examples include the dural venous sinuses, which transport deoxygenated blood from the brain to the heart, and the coronary sinus of the heart.